Metal oxide production



1954 c. M. OLSON ET AL 2,670, 7 METAL OXIDEK PRODUCTION Filed Sept. 2. 1950 ""cowmzzmva GAS I I I I I I, I, I I I I 1 I 1 1 1 1 1 11 111/11 1 I I III ii H 220 6Z2 PRODUCT INVENTORS CARL M. OLSON 8: JOHN N. TUL LY J BY kit-M ATTORNEY Patented Feb. 23, 1954 I METAL OXIDE PRODUCTION Carl M. Olson, Newark, and John Nelson Tully, Wilmington, Del., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application September 2, 1950, 'Seriai No. 182,922

(Cl. 23- -202i Claims.

1 This invention relates to the production of metal oxides through the vapor phase oxidation orhydrolysis of their halides, and more particularly to the preparation of pigmentary, finelydivided titanium oxide by reaction of an oxygencontaining gas with vaporous titanium tetrachloride.

It is known that metal oxides may be'produce'd by the oxidation or hydrolysis of vaporized metal halides. 'In such processes a volatilized metal halide, such as a chloride of iron, titanium, silicon, aluminum, etc., is reacted at elevated temperatures with oxygen, air, or like suitable oxygen-containing gas to form the respective oxide of the metal and chlorine. Similarly, the volatilized metal chloride can be reacted with steam from which the desired metal oxide and hydrogen chloride is obtained.

In such prior processes, a portion of :the oxide product formed deposits upon and tenaciously adheres to the internal surfaces .of the reactor,

instead of being swept out of such reactor by the gas stream. This is very objectionable for several reasons. Among other reasons, a :portion of .the powder is thereby lost, since it frequently packs or sinters to such an extent that it cannot be recovered in useful form. Again, if the oxide forms in a desirable state, the difierent conditions prevailing at the surface of the reac'tor over those existing within the reaction space cause an undesired particle size growth and crystallinity change which renders the product unfit for an intended or pigmentary use. Even where loss of material can be tolerated, segments thereof frequently dislodge from the reactor walls and contaminate the desirable products being recovered from theoperation. A further objection resides in the undesired alteration in the configuration of the internal surfaces and dimensions of the reactor which the presence of the deposited mass induces. In processes of the type mentioned, equipment design is frequently a critical factor and even minor dimensional changes cannot be tolerated. Fur- :thermore, if reaction product build-up proceeds :for too long a period, the vessel will be completely blocked off or plugged and shut-down for dismantling and clean-outis then necessary. A still further disadvantage is the inhibiting effect on heat transfer which the presence of the 2GB- posited solid exerts. It is often necessary to 'add to or remove from the reaction mixture "substantial quantities of heat, which isconveniently accomplished by means of heat transfer through the reactor'walls. Obviously, 'thepresence ofa relatively thin layer of solid, non-conductive reaction productupon suchwalls is often sufiicient to retard heat flow to a. considerable, undesired extent, especially when such solid is in finelydivided. state and interspersed with minute .gas pockets.

A wide variety of proposals for removing deposits =of :type has been suggested, including mechanical scraping. Unless the equipment is shut down during the scraping, the presence of corrosive substances at elevated temperatures renders the choice of suitable materials of con-- strnction, or design .of operable .stuiiing boxes, -:.extremely diiiicult. Furthermore, the scraping must be effected at extremely frequent intervals, or the deposit will become .so hard and adhere so tightly to the .reactor surfaces that the scraper will. either fail to dislodge it, or damage the surface .in doing so. Contamination of product with fragments of build-up :is an attendant disadvantage. Chemical. removal :can be resorted to, but this ordinarily requires expensive shutdowns with considerable .loss of time being incurred in the process. Careful design of equipment to minimize to some; extent the opportunity for forming objectionable deposits can .be' resorted to, but in most cases equipment design is so rigidly prescribed byxthedemands of the process itself that variation therein for the purpose of preventing such build-up results in objectionable loss of yield, reduction in product quality, and increased-costs. .Ithas been proposed to'.introduce the halide through a shielding gas envelope, so that the reactants do :not mix until they are in the central portion of the reaction vessel. While it is theoretically possible to eliminate wall build-up by arranging jets which willsweep all the surface of the vessel with a stream of gas, for example air .or chlorine, this procedure has the disadvantage of requiring enormous quantities :of shielding gases. U11! fortunately, :in obtaining products in desirable .form it is essentialthat reactantsbe mixed rapidly in a confined space and diluted :as little-as possible, requirements which-are diametrically opposed to the :needs of :a gas shielding type :of operation. Formation :of tightly-adhering deposits of coarsely crystalline material may :be prevented for a relatively :short period by maintaining the reactor walls .at temperatures below those at which :the reaction takes place. This procedure, however, does not prevent formation of an insulating layer of oxide product or pigment, so that-objectionable coarse material soon form-s anyway.

From the foregoing, it is evident that, for economy of operation and production of oxide materials of optimum characteristics, a real need exists for a useful, workable method for preventing or minimizing the formation and deposition of tightly-adhering metal oxide upon the surfaces of a reactor exposed to the vapor phase oxidation or hydrolysis of a metal halide in which such oxide is produced.

It is accordingly among the principal objects of this invention to overcome the disadvantages which attend prior methods for the vapor phase oxidation of metal halides and to provide a novel and effective method and apparatus for accomplishing such objects. It is among the particular objects of the invention to effect the production of valuable forms of pigment-useful metal oxides without attendant formation of objectionable oxide deposits on the surfaces of the apparatus employed in such production, and to provide a novel form of apparatus which may be operated continuously, over lon periods of time, and which in no way limits the choice of conditions which are favorable to production of maximum yields of optimum quality products. A further and special object is to provide a relatively simple and effective method and apparatus for continuously manufacturing pigmentary titanium dioxide through the vapor phase oxidation at elevated temperatures of titanium tetrachloride while avoiding formation of objectionable oxidic scale formation and deposition and resulting apparatus plugging which has characterized prior TiCli oxidation processes. Other objects and advantages of the invention will be apparent from the ensuing description and accompanyin diagrammatic drawing, wherein the figure is a vertical, sectional view of one useful, illustrative form of apparatus suitable for adapting the invention to practical application.

These and other objects are attained in this invention which comprises vapor phase reacting a metal halide with an oxidizing gas within a confined reaction zone comprising a substantially solid, porous refractory material, and during said reaction diffusing a gaseous medium into said zone through said material sufficient in amount to provide a protective gas film over and shield the internal surfaces of said zone from substantial contact with said reactants.

In a more specific embodiment, the invention comprises reacting, preferably at temperatures in excess of 1000" 0., vaporized titanium tetrachloride with a humidified oxygen-containing gas, effecting said reaction within a restricted,

tubular conduit comprising a solid, porous refractory substance, and throughout said reaction maintaining a layer of an inert gas within and over the internal surfaces of said zone by diffusing said gas slowly and continuously through said substance from an extraneous source maintained about said zone whereby said surfaces become shielded from and are maintained out of contact with said reactants.

Referring to the embodiment of the invention illustrated in the accompanying drawing, there is shown a vertically-arranged furnace i which can be wholly or partly insulated and heated in accordance with conventional, desired means. Suitably disposed within the heating chamber 2 of the furnace l is a tubular element 3, composed of corrosion-resistant metal or other suitabl and desired material, having an inlet 6 and an outlet through which reaction products are withdrawn for recovery. Substantially intermediate the length of said tubular element, a reaction zone 6 is provided, the defining side walls I of which form, as shown, a continuation of walls 8 and 9 of tubular element 3. The reaction zone walls 1 comprise a porous, refractory material such as porous (unglazed) porcelain, silica, nickel, steel, etc., or other suitable solid substance adapted to enable an inert gas (nitrogen, carbon dioxide, helium, argon, etc.) to readily diffuse therethrough and for a purpose and in a manner to be presently referred to. Suitably disposed above the porous, tubular reaction zone 6 and in concentric, spaced relationship about tubular element 3, is a second tubular element ID. This is also constructed of corrosion-resistant metal or other material adapted to withstand relatively high temperatures and the corrosive action of fluids subjected to reaction within the apparatus. The element H3 is provided with an inlet ll communicating with a. passage l2 and terminates as an annular discharge or slotted jet outlet 13, formed by interpositioning, as shown, portions of tubular element 3 in close proximity to each other. Said outlet l3, passage [2 and inlet II are maintained in open communication with the interior of conduit 3 as well as reaction zone 6. concentrically arranged in spaced, substantially enveloping and gas-tight relationship about the cylindrical, porous walls 1 of the porous, tuoular reaction zone 6, is a cylindrical element 24, also made up of corrosion-resistant metal or like material, Which forms a passage l5 and is provided with an inlet 16 through which an inert gas from a source of supply (not shown) can be constantly maintained at any desired pressure about the said porous walls i and reaction zone 6.

The operation of an apparatus of the type de scribed will now be detailed and with particular reference to one preferred adaptation of the invention in which production is effected of titanium dioxide, in accordance with, for example, the methods disclosed in U. S. Patent No. 2,488,439, dated November 15, 1945, to Holger H. Schaumann. In that atent rutile or anatase pigmentary TiOz is obtained by decomposing in the vapor phase a titanium halide, such as titanium tetrachloride, with an oxidizing gas such as oxygen, air, oxygen-enriched air, or mixtures thereof with various inert gases, and in the presence of regulated, small amounts of water vapor. In such decomposition, temperatures ranging above 800 C., and particularly in excess of 1000 C., and up to, say, 1350 C. or 1450 C., or

higher, are resorted to. In applying such a method to this invention, the oxidizing gas, suitably humidified with from, say, about 0.05% to 10%, and preferably from 0.1% to 5% (based on the total volume of gases) of water vapor, is charged separately and continuously into the furnace l of the reactor through tubular conduit 3 and via inlet :2 thereof to ultimately pass and discharge into the porous walled reaction zone 6. During such passage, heating of the oxidizing gas to any desired preheat or reaction temperature is effected, and immediately prior to its injection into the reaction zone 6 it becomes rapidly mixed with vaporized, anhydrous titanium tetrachloride being concurrently fed, continuously and separately, to such reactor and from a source of supply (not shown) via its inlet H, passage I2, and the annular slot jet means 13, so that it charges into the oxidizing gas, passing through conduit H3, in the form of a relatively thin sheet or stream, preferably in a direction at right angles to and across the axis of flow of such gas. The

Faitnt 2,488,439, aaa the pigmemery 1 102 1 ebbiit aria 'exteriorly surrounds the porous Wall 1 1 of the tubular reactor 6. Preferably, the inert gas is maintained withinsaidpassage under an elevated pressure at least slightly inexcess of that prevai'ling within the reaction zone 6 and adequate to force or cause the gas to slowly and steadily flow or diffuse through the pores of said wan "-1 into the interior ofthe reaction zone. As a -result, a protective film or layer of an insulating 'o'r shielding gas is formed over and upon the internal surfaces of said zone andeffectively prevents or minimizes contact of said. reactants or their resulting reaction products with the internal surfaces or the reactor to avoid objectionable "oxide scale deposition or bu'ild up thereon.

To a clearer understanding of the invention, the following specific exam 1esaregaen, which are merely illustrative "cff sesame applications of the invention and ar not 'to be "construed as in limitation of its underlyihgscopeand effect:

In this example, a reactor of tae= errra1strea tu're shown in the accompanyin -a'wiiag's was efiipmyed, 'in which the etirees, t'ub far-reaction 7" in length, aav g A and 2" "outer d1 meter, i ii an ave ge p'ore "radius of -22 microns. lhis tilbulai' element enclosed in a fused silica jacket of *the same length having an internal diameter -of3%" and provided'with twoperforated silica tubes for the even distribution of the Shielding gas within the space separatin the-porous tube and its jacket. Thecfr'cuinfrential jet or slotted inlet was enclosed within the furnace -which Was' inaintained at 1020" 'C., but the porous tubeassembly was *eapbsea to the atmosphere for cooling purposes. Dry nitrogen at C. was passed into the jacket space and forced through the walls of the porous tube "into the reaction zone at the rate of 3-7.6 g-i a m mol'es per hour. The reactants were-separately introduced into the'reactor through silica coils in a pfel'ieating chamber also maintained at 1020 C., and at the following rates i'n grarn -mols per hour:

After 45"minutes of operation the tube was inspected and it was found that thebott-omfi fl were completely free of oxide deposit. The top -V of the porcelain tube held i=6 grams of I coarse, crystalline Erma-representing 0.5% 0f the T102 thruput, this being attributable to the fact that this portion or the tii'be 'was heated are ts being aara een 'the rernace. eszeeueia creamy pi inent was ebta a, a piete rea'enea, "product exhibiting serenetjvryiueiag power; tiatisgstreaeth, arm aerate ursrenmty.

Example I was duplicated, usinfi theis'ahie peratures and flowrates'of. reactants butemploying a porcelain tube element 1 I. D. x 2" G). D. 11% ions, with average pore reams. "of microns, through the wane of 'ivh'ibh nitrogen was forced at a rate bf 355 "grain 'in'ols per ne'er. --A'tei"72riiif1'ute s of opeiation ah auauty pigment was reeevere'i the of the porous tube "was to contain 1 3 atan'setcrystalline dep'c mit;{the' bottom {1 e-ram 'e'f o t 'p' fifent deposit; ana the central totalefi iiiit esteemeposi-t at' top anh bottom resaame nom the se'alrag of "the porous Wall {by use bf "the jacket which would; be avoided in lareer sifi equipment construction.

I In-an' apparatus of the general eonstmotion shown in the drawing but containing -'a porous refractory reaction zone tube, 1" I=. i 'x l*: 0. D. x 3" long, having 'anavrageipore radius "of 2.2 microns, fitted with a fusetl' 'silioa jacket having an internal diameter of 2%, and in whichrthe porous tube assembly was exposed to the atmosphere for thezpurpose of cooling "7 mole per hour each. of chlorine and earbon monoiide were passed. into the jacket :pa'ssageand through the walls of the .porous tube, at a temperatureof 20 0. Oxygen and TiCh reactants' were separately introduced to the reactor, being added via a preheating chamber and. [jet maintained at 10050gand "at the following rates in g'ram 'inols per hoii-r:

02 (containing 1.1 mols/hour HzO'),- 18

After 10 minutes, the run -was discontinued tb inspect the condition of the interior of porous tube. No observable oxide depositwas round'to exist on its internal walls.

In a comparable operation to the above examples, but in which a non-porous silica tube 'reactor was used as "a control instead of a poreus typ'e tube in conjunction with an inert gas, aria both the .jet and reactor placed in a furnace at -I000"-C., it was found that after'only la-minutes of operationall parts of theintei nal wall'sof the reactor were covered with coarsely crystalline titanium 'oxidesc'alefwhich weighedin the aggregate 44 grams,or--l0i5'% of 'tHeTiOz 'thruput.

Difficulties with build up are most {severe in small-scale experiments such as those ees'cribea. -In plant scale equipment, where the -ratio "cresposed surface to the "vomme er 'r'eacta s', afic'lj to the weight 'of products is -"grea'tly diiiiinish'edfi'the invention will 'be reune to be' mos't 'iisefully ei fec- "tiVe. Fhe following example demonstrates the value "of the in ven'tion in-"a larger size -'oxidatio"n plant.

An apparatus oi the g'enei-al structure shown in the drawing was employed,*=with-a;porous silica refractory tube, having an average spore =-radius of- -2 -niicrbns, and 5 D. -x g 2 7", long, being 7 lowing rates, in gram mols per hour:

TiCh 56? Dry air 870 Moist air 2100 (providing 28.4- gram mols per hour H2O) into the reactor through a cooled jet, maintained at 200 C. The moisture for the moist air stream was added by burning hydrogen in oxygen in the line just ahead of the jet, so that additional heat was provided. After one hour of continuous operation, the porous tube was inspected, and was found to contain only light traces or dusting of soft, pigmentary TiOz on portions of its lower third, which would in no way have hindered continued operation.

A similar and comparable run was made but with a fused, non-porous silica tube having the same dimensions as those of the porous tube used above. In this instance it was necessary to stop the reaction at ten minute intervals to scrape out the silica tube, to removedeposited T102 and in order to enable the run to proceed. After a net running time of one hour, further operation had vto be discontinued due to hard scale formation and build-up on the internal surfaces of the silica tube which the scraper could not remove without danger of damaging the tube. ihis build-up in the run was found to total 8 92 of the TiOz throughput.

EXAMPLE v Example I was duplicated, except that air preheated to 890 C. and containing .95 E20 by volume was employed as the oxidizing gas, and was continuously charged at a rate equivalent to 19 parts by weight of 02 per hour through the reactor. Simultaneously vaporized TiCli at a temperature of 865 C. was continuously charged at a constant flow rat of 100 parts by weight per hour to the reactor and issued from its peripheral slot jet inlet as a sheeted stream and across the air stream being conveyed to the reaction zone. Instantaneous reactant mixing thereby took place with complete reaction ata temperature of i150 C. occurring in the reaction zone and with an average retention time of reactant gases therein being only .13 second. The T102 suspension issued from the reactor outlet at a temperature of about 1000 C. and was immediately quenched (within 2 seconds) to 300 C, by commingling therewith sufi'icient cold chlorine gas. The 'IiOg product was then separated and recovered from the cooled reaction products in a conventional type filter, and comprised a highgrade rutile pigment. As a result of this operation, substantially 190% 'IiCli conversion was found to take place, and upon inspecting the interior of the porous tubular member employed as the reaction zone at conclusion of therun, its surfaces were found to be free of any objectionable oxidic scale deposit.

EXAMPLE VI Example I was duplicated except that in lieu of the porcelain reaction zone employed in the reactor assembly of that example, use was made .of a .1 thick porous nickel tubular element having an average pore radius of 25 microns and of equal length and internal diameter with the porcelain tube. A A" tubular nickel coil was disposed about the exterior of this porous tube, and a heat transfer oil cooling fluid was continuously circulated therethrough to maintain the porous wall reactor in relatively cool state during the reaction. The porous nickel element and cooling coil were enclosed within a fused nickel jacket element, the internal diameter of which was 4" and in which perforated nickel tubes were provided for promoting even distribution of the shielding gas within the space separating the porous tube from said jacketing member. In the operation the dry nitrogen was passed into the jacket space and forced through the walls of the porous nickel tube into the reaction zone at a rate of 42 gram mols per hour. After .25 minutes of operation, substantially the same results as those shown in Example I were realized.

Although the invention is described in the above examples as applied to certain specific embodiments wherein particular reactants, temperatures, retention times, ratios, velocities, apparatus, etc., have been employed, it obviously is not limited thereto. It will be understood, therefore, that variance therefore can be resorted to without departing from its underlying principles and scope. Thus, while the invention is outstandingly useful for converting TiCli, through oxidation, to TiOz to obtain a high-quality pigment therefrom, it is also utilizable in the oxidation of other titanium halides or mixtures, and especially the chlorides, bromides or iodides of that metal, as well as those of such additional metals as zirconium, aluminum, silicon, antimony, tin, zinc, etc., which react in the vapor phase with an oxygen-containing gas of the type herein contemplated to produce a solid, preferably white, metal oxide.

Again, while air, suitably enriched with H2O in the amounts mentioned, comprises a preferred, useful form of oxidizing gas, if desired, other types and amounts of oxidizing gases or mixtures thereof, in either dry or humidified state, and such as those already mentioned, can be employed.

Normally, the oxidation reaction is conducted at atmospheric pressure, but, if desired, it may be effected at super or subatmospheric pressures, and the differential in inert gas pressure can be correspondingly varied. Similarly, any type or size of reactor conforming to the intended scale of operation can be used in applying the invention; and while resort to a continuous operation is preferred, a discontinuous or batch type of procedure can also be adopted, if desired, The time of retention of reactants and resulting oxide product within the reaction zone is quite critical and important where pigmentary 'IiO2 is being produced. In such instance, reactant retention times ranging from about .01 to 5 seconds are generally resorted to, with a preferred time ranging from .1 to 1 second. With such retention times, undesired particle size growth due to sintering of aggregates is avoided, which, if allowed to take place, would deleteriously aifect ultimate IiOz pigment properties, especially tinting strength.

In the accompanying Fig. 1, which is a diagrammatical view of a simplified apparatus for carrying out one embodiment of the invention, the reaction vessel comprises a cylindrical porous r -ears 9. irae ervtu ee. ea ed we a ass-fi ht iaelsetes rl mi r flaaer havin n e e the. ma s aaee- M s"t e neeor T e see ress s his. s ia sl htl higher 11 hat zone itself, so

. an at su h me a "a. a si e ee inlet atat elie a the wm s.

wee aler eati as th s may ne af e e emete an adeeue e i tr tiee e t e. P e e esi f-h efi t li s at the s eldin film wi be. te -ed: e QeP nel n i s eer imi t v r t e 1 tire reaction ne. satiaee s be ere eet siln it ri frequen l b e le ad ntageous e eest t9 meaiield refieeti r app t nin e as t eree ea 0 that nr eetie Of a l ar s. f. e eee ee wil be assur s ev n h ei uant ty of h e d n s Ales th u h the eee mnenrine drawin t e nveatien as been, l ed o l to the r eet the eeaeiie sse a Z ne itse b ou l t pr nc e new b e nd d o he preteetien ef he inle iet as w l. ren e e r aeter ur a s whieh. ebie tienab e xi de o i a u It rertieular r e nven en a p a us such as that to extend the porous tube up ate th J t. so. that t. termin es to orm ne ide o th lot iet nlet t ou h wh h e um tetra hler de s. adm t If bu d-up hould he. eme nt red en. o h a es o a iv all. doubl serous wall. spaee to vide a a are o eha ne e the i t oduc o o th i time easean e emplo d- Q her possible .ene n. the pa atu illu tra ed w e.- ems lr s to tho s lle in the a As alread indicate manyl ie e t types of errmateria s a e ui ab usef l on, Thee eiee o a p t cu ar 3 p.- dep n upon u h con ons as em ra r ree of reacta expo ure to cor.-

res viee a and me a ica tre h re: qui d. The size range, uniformity of size and distribution of pores in the refractory material, nd. Si e f the m nute pa t tions between t e pores. will ohyiously affect the distribution of the shielding gas and consequently the ease with whi h pre et e fi m i ma n ained Within the reactor. As noted, porous tyne porcelain or cerarnic bodies are generally utilizalole herein as a sula serous oxidizin chamber, a a e porous iee 0. nere s m tal i bodies pemrosed of 10 at a temperature adequate to bond the crystals together, but insufficient to convert the crystals to flowing condition. Thereafter, the sintered product is cooled and hardened. Examples of conventional methods for preparmg such refractories, comprise those referred to, commencing at page 167 of Powder Metallurgy by P. Schwarzkopf published 194]? by MacMilla-n Company. The

minimum usable pore size depends on the size of the gas molecules to be passed through the refractory. For exarnpfe, when helium is used as the inert gas, a smaller. pore sizev is employed than when carbon dioxide is used. Generally an average pore radius of less than microns is e r e e re ere l P re an being t -eemi ree' e 2, m en f j iie her a reetl fli rre e e e esiree 'i u v abl o of larger or less uniform pore size can log, 9? W b applying a P 39 4 coating r t 3.

as by burning a rnetal ha ide within the element,

he nne e e e of h el i te' be P ete' ed a b s r n r filterns a e as est e ih ee i n a s ta flui ee u heer eee- The fi je j f e size i thee b ee ermiaee b ti e ea er-e of the applied material.

i 59 1 Wa made 9 man f rent ea i9 ei ee ea r e f e ma sh e din fil s? the choice of gas being dependent upon t ns needs of a particular application. generally, any medium or mixture thereof which is gaseous at "normal temperatures or pressures can be resorted to in the contemplated shielding operation. Examples thereof include air, oxygen, nitrogen, carbon dioxide, chlorine, eto., as well as the members of Gr p Ze 9f the rerieslie s s e hel um erase reaction products may serve as acomponent of the shieding gas or the mixed product gases of the reaction may be recycled for use in coniunction with the inert gas. 'Thus. in the co-pen'ding application Serial No. 658,868, of Kraus and Schaumann, filed April 1, 19.46, removal of oxide build-up by means of gases which chemically combine with the oxide is disclosed. Adaptation of that method for use in conjunction with the present invention to insure a continuous process affords an effective method for preventing scalo formation, since any particles which might in someway penetrate the gas'film will be quickly removed by dissolution. A particular embodiment of the invention of that application involves the use of a mixture of carbon monoxide and chlorine for the vapor phase oxidation of tita nium tetrachlorida' Titanium dioxide particles which penetrate into the gaseous f lm issuing from the porous wall will be'chlorinatad accord ing to the equation:

r oazeiazee aeiaze e chlorine, of course, is reeoflilered along with the lar application, since it depends on the shape and size of the reaction vessel, velocity, temperatures, composition of the reacting gases, nature of the shielding gas, uniformity and degree of porosity of the porous wall, the degree of protection from deposit required, and many other factors.

In some applications it has been found advantageous to cool the porous surfaces which are to be protected. If they are near the temperature at which the reaction takes place, a fine crystalline deposit plugs the pores when the flow of a given shield gas is equal to that which is sumcient to give complete protection when the surface is not heated. If the process demands that the surface be hot, a larger gas volume can of course be used, which will afford adequate protection.

A comparison of Examples I and IV and their control runs demonstrates that, although dilution of the product gases by the shielding gas in a small unit of the type used in Example I is appreciable, it is substantially less in a larger unit of similar shape, due to the decreased ratio of wall surface to reactor volume. Hence, dilution in a plant size unit will be found to be substantially negligible. This is illustrated by the following table:

Concentration of chlorine in product gases It should be understood that the above is not based on the minimum possible flow of shield gas, but merely on quantities which it has been found convenient to use in the experiments referred to. Also, the concentration of chlorine may be increased, rather than decreased, if employed as a component of the shielding gas.

It will be apparent that the invention can be readily adapted to fit the particular needs of any variation of the oxidation or hydrolysis processes for the production of metal oxides. By its practice, prior expensive and difficult batch types of operation can be readily converted to an economical, continuous type of process and without loss of yield through scale formation or the experiencing of shutdowns through apparatus plugging. Moreover, the practice of this invention does not appreciably affect the reaction itself; so that its full potentialities for the economical production of new and valuable forms of metal oxides are realized.

We claim as our invention:

1. A method for producing a metal oxide which comprises decomposing a halide of a metal with an oxidizing gas in the vapor phase within a restricted reaction zone, and during said decomposition maintaining a shielding inert gas film over the internal surfaces of said zone adapted to prevent substantial contact of the reactants with said surfaces, said shielding gas being introduced into said zone through a porous refractory wall of said zone and from a source maintained about its exterior.

2. A method for producing titanium oxide which comprises decomposing a titanium halide with an oxidizing gas in the vapor phase within a restricted reaction zone, and during said decomposition maintaining a film of an inert gas over the internal surfaces of said zone adapted to prevent substantial contact of the reactants with said surfaces by diffusing said inert gas through a porous refractory wall into said zone and from a source exteriorly maintained about said zone.

3. A method for producing titanium oxide which comprises decomposing titanium tetrachloride with an oxidizing gas in the vapor phase within a tubular, porous type reaction zone, and during the decomposition maintaining a film of an inert gas over the internal surfaces of said zone to prevent substantial contact of the reactants with said surfaces by flowing said inert gas through the porous walls of said zone from an external source maintained under pressure about the exterior of said zone.

4. A method for producing pigmentary titanium dioxide which comprises decomposing titanium tetrachloride with a humidified oxidizing gas within a tubular type reaction zone composed of porous refractory material, eifecting said decomposition at temperatures ranging from at least 800 C. to about 1450 C., and throughout said decomposition maintaining a film of an inert protective gas over the internal surfaces of said zone to prevent the reactants and products of reaction from coming in substantial contact therewith by diffusing said inert gas from an external source surrounding said zone through said porous refractory material.

5. A process for producing a titanium oxide pigment which comprises reacting in the vapor phase titanium tetrachloride and a humidified oxygencontaining gas at a temperature in excess of 1000 C. and within an oxidizing zone the walls of which comprise a porous refractory material, effecting said reaction over a reactant retention period within said zone of from .01 to 5 seconds, and throughout said reaction maintaining a film of an inert gas over the internal surfaces of said zone to prevent substantial contact of said reactants with said surfaces by continuously diffusing an inert gas into said zone through said porous refractory material and from a source which is exteriorly maintained about said zone, and recovering the resulting T102 product.

6. A process for producing a titanium oxide pigment which comprises reacting, at temperatures in excess of 1000 C., vaporized titanium tetrachloride with a humidified oxygen-containing gas, effecting said reaction within a reaction zone comprising a porous refractory tubular conduit, and throughout said reaction maintaining a layer of an inert gas within and over the internal surfaces of said zone by slowly diffusing said inert gas from an external source through said porous refractory whereby said surfaces are shielded during the reaction from direct contact with said reactants and thereafter recovering the resulting TiOz product.

7. A method for producing a titanium oxide pigment which comprises reacting in the vapor phase, and at temperatures in excess of 1000 C., titanium tetrachloride and air containing an amount of water vapor ranging from about to 10% by volume, based on the total volume of gases being reacted, effecting said reaction within a refractory reaction zone comprising a tubular conduit of porous refractory material and over a reactant retention period therein of from taining a layer of a non-reactive gas over the intemal surfaces of said zone to shield them from contact with said reactants by continuously diffusing said non-reactive gas through said porous material from a source maintained under pressure about and surrounding the exterior of said reaction zone, and recovering the resulting TiOz pigment.

8. A method for producing titanium oxide which comprises subjecting titanium tetrachloride to decomposition in the vapor phase with an oxygen-containing gas in a confined reaction zone made up of a porous ceramic material, and during said decomposition shielding the internal surfaces of said zone from substantial contact with the reactants by diffusing through the walls of said zone from an external source a sufficient quantity of an inert gas to form a protective film over said surfaces.

9. A method for producing titanium oxide which comprises subjecting titanium tetrachloride to decomposition in the vapor phase with an oxygen-containing gas in a confined reaction zone made up of porous silica, and during said decomposition shielding the internal surfaces of said zone from substantial contact with the reactants by diffusing through the walls of said zone from an external source a sufficient quantity of an inert gas to form a protective film over said surfaces.

10. A method for producing titanium oxide which comprises subjecting titanium tetrachloride to decomposition in the vapor phase with an oxygen-containing gas in a confined reaction zone made up of porous nickel, and during said decomposition shielding the internal surfaces of said zone from substantial contact with the reactants by difiusing through the walls of said zone from an external source a sufficient quantity of an inert gas to form a protective film over said surfaces.

CARL M. OLSON. JOHN NELSON TULLY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,685,759 Walter Sept. 25, 1928 1,881,041 Benjamin Oct. 4, 1932 1,967,235 Ferkel July 24, 1934 2,062,358 Frolich Dec. 1, 1936 

1. A METHOD FOR PRODUCING A METAL OXIDE WHICH COMPRISES DECOMPOSITION A HALIDE OF A METAL WITH AN OXIDIZING GAS IN THE VAPOR PHASE WITHIN A RESTRICTED REACTION ZONE, AND DURING SAID DECOMPOSITION MAINTAINING A SHIELDING INERT GAS FILM OVER THE INTERNAL SURFACES OF SAID ZONE REACTANTS TO PREVENT SUBSTANTIAL CONTACT OF THE REACTANTS WITH SAID SURFACES, SAID SHIELDING GAS BEING INTRODUCED INTO SAID ZONE THROUGH A POROUS REFRACTORY WALL OF SAID ZONE AND FROM A SOURCE MAINTAINED ABOUT ITS EXTERIOR. 